Vessels molded from plastic materials such as polyethylene terephthalate (PET) and the like and each having a neck over which a screw cap can be mounted are currently used for soft drinks and other beverages.
The soft drinks and particularly soft drinks with fruit juice contained therein are generally sterilized with high-temperature before they are charged into plastic vessels. Thus, the plastic vessels are required to have a sufficient heat resistance. Particularly, the non-stretched mouths of the plastic vessels must have a higher heat resistance such that any heat distortion will be prevented since the screw caps are to be fitted thereover.
One of the plastic vessels having their mouths which fulfill the above requirement is proposed, for example, in Japanese Patent Publication No. Sho 61-35056. As shown in FIG. 12, this technique provides a plastic vessel 1 which comprises a biaxially stretched vessel body 2 and neck portion 5. The neck portion 5 includes a thread 3 formed on the outer periphery thereof and a support ring 4. The heat resistance in the neck 5 is increased by once heating it up to a preselected temperature and then gradually cooling the neck to whiten and crystallize it. The plastic vessel 1 has an advantage that the neck portion 5 thereof is difficult to he distorted at raised temperatures and can maintain its dimensional accuracy at higher levels.
However, such a prior art has a problem in that if the degree of crystallization at the neck portion 5 is to be sufficiently increased, a portion or the vessel body 2 adjacent the neck portion 5 can be undesirably crystallized since the neck portion 5 is continuously connected to the vessel body 2. In other words, a portion of the vessel body 2 adjacent to the neck portion 5 will be undesirably crystallized when the neck portion 5 is crystallized in the preform forming step. The crystallized portion of the vessel body 2 becomes difficult to be biaxially stretched in the subsequent step. If the neck portion 5 is crystallized in such a step that the plastic vessel 1 has been biaxially stretched and blow molded, even a portion of the shoulder or barrel of the vessel body 2 in addition to the neck portion 5 can be crystallized and whitened.
Japanese Patent Publication No. Hei 2-5564 or U.S. Pat. No. 4,715,504 discloses a preform comprising an outwardly threaded insert piece made of a plastic material having an increased heat resistance and the neck and body portions of the preform other than the insert piece formed of another plastic material such as PET or the like. Such a preform is formed by first injection molding the insert piece from the increased heat resistant plastic material and then injection molding the PET material so that the insert piece will be embedded into the outer periphery of the neck portion.
The preform formed by the use of such an insert piece can overcome the aforementioned problem associated with the undesirable crystallization of a portion of the vessel body when the whole neck portion is crystallized and increase the heat resistance of the neck portion to a practically satisfactory level.
However, the use of the insert piece does not necessarily provide a sufficient bonding strength between the insert piece and the preform body. In the worst case, the insert Piece may be partially separated from the vessel body due to lead on fitting the screw cap.
In order to overcome the above problems in the prior art, the present invention has its object to provide a preform having an increased mechanical strength which comprises an insert piece formed of a plastic material having increased heat resistance and mechanical strength, such a plastic material being firmly bonded to another plastic material forming the inner layers of the preform body and neck portions, and also to provide a mold usable in molding such a preform.